This invention relates to a new and useful process for reducing the levels of t-butyl alcohol (TBA) produced during the visbreaking of polyolefins.
The visbreaking of polyolefins to improve their rheological properties is well known in the art. Although this process of visbreaking can and does occur naturally at appropriate temperatures, the common and wide-spread industry practice is to induce and/or accelerate the process by means of suitable chemical reagents. It is also well known in the art that peroxides are the chemical reagents of choice used in the visbreaking process. This is because these peroxides are capable of generating free radicals at the appropriate temperatures and times so as to ensure the production of a uniform product.
The use of peroxides in the visbreaking process is succinctly discussed in Ehrig et al (U.S. Pat. No. 4,707,524): That discussion in Ehrig et al., subtitled Background Art, is hereby incorporated into this application.
It is common knowledge within the plastic industry that 2,5 dimethyl 2,5-bis(t-butylperoxy) hexane, commercially made and sold under the trademark "LUPERSOL 101" by the Lucidol Division of Pennwalt Corporation, is the preferred peroxide for use during visbreaking processes. In spite of its excellent properties and the improved rheological properties of the visbroken polyolefins, Lupersol 101, has some drawbacks. The primary drawback is the production of high levels of t-butyl alcohol. This in turn results in undesirable odor problems, and in some cases prevents the use of such visbroken polypropylene in food areas. Prior to July 1987, the maximum allowable level of t-butyl alcohol in such visbroken polypropylene permissible in food areas was less than 25 ppm. Although the present maximum allowable level of t-butyl alcohol of visbroken polypropylene permissible in food areas has been relaxed to 100 ppm, it is still desirable to eliminate TBA in food areas.
The use of zeolites as sorbents and as catalysts is also known. Starks, (U.S. Pat. No. 3,283,015) discloses the use of zeolites as sorbents to remove alkandiols which are impurities of alkanols. The use of molecular sieves/zeolites as catalysts is also disclosed in the following references--R. Barrer, Zeolites and Clay Minerals as Sorbents and Molecular Sieves p. 5-14 (1978); Vol. 15. Kirk-Othmer, Encyclopedia of Chemical Technology p. 650-651 3 ed. (1981). (Discussion on Molecular Sieves referenced pages on Acidic Zeolites and Zeolite Catalyst Applications).
The elimination and/or reduction of the t-butyl alcohol produced during the visbreaking of polyolefins continues to be of interest to the plastic industry. Ehrig et al represents one such attempt. However, the Ehrig et al method merely avoids the problem by resorting to the usage of other peroxides--"LUPERSOL 553 and "ESPERAL 529"--other than "LUPERSOL 101". (See lines 20-24 column 2 of U.S. Pat. No. 4,707,524) It is uncertain whether this process is economically desirable considering cost and availability of substitute peroxides and adjustments in process conditions and equipment that such substitutions may necessitate. It is also uncertain how many of the improved rheological properties of the visbroken polypropylene are compromised by such substitutions. For example, in Table II, page 3, of Ehrig et al; it is observed that the melt flow (MF) of resins treated with other peroxides is significantly less than that treated with Lupersol 101.
The present invention represents a new and significant improvement in the visbreaking of polyolefins. It permits the continuous and preferred industry use of Lupersol 101 as the peroxide of choice, while significantly lowering the undesired production of t-butyl alcohol that is an inherent by-product of the process.